The present invention relates to an image scanning device which irradiates light from a light source which can move along a document table onto a document sheet placed on the document table, and outputs an electric signal corresponding to the reflection light.
Conventionally, a scanner which irradiates light from a light source provided in a scanning unit which can move along a document table onto a document sheet placed on the document table, and outputs an electric signal corresponding to the reflection light by a CCD line sensor, is put to practical use.
In such a scanner, the light source is moved from its home position over a read area of the document sheet placed on the document table in the sub-scanning direction of the document by means of a carriage. During the scanning, the moving speed of the carriage varies depending upon the magnification by which the original copy is read.
Let us take examples where an original document is read at read magnifications varying from, for example, 50% to 400%. The moving speed of the carriage when the read magnification is 50%, 100% or 400%, will now be described with reference to FIG. 1.
When the read magnification is 50%, the movement of the carriage is accelerated (for time T3) from the home position HP until it reaches a moving speed 1 (in this example, 400 mm/s) for reading the original document at a read magnification of 50%. Then, while the speed 1 is maintained, the carriage passes the image scan start position (that is, light irradiation start position) on the document table, and thus the image on the sheet placed on the document table is read.
When the distance from the home position to the image scan start position is set to 60 mm, the carriage reaches the image scan start position in 190 ms as shown in FIG. 1.
When the read magnification is 100%, the movement of the carriage is accelerated (for time T2) from the home position HP until it reaches a moving speed 2 (in this example, 200 mm/s, which is ½ of the moving speed 1) for reading the original document at a read magnification of 100%. Then, while the speed 2 is maintained, the carriage passes the image scan start position (that is, light irradiation start position) on the document table, and thus the image on the sheet placed on the document table is read.
When the distance from the home position to the image scan start position is set to 60 mm, the carriage reaches the image scan start position in 320 ms as shown in FIG. 1.
When the read magnification is 400%, the movement of the carriage is accelerated (for time T1) from the home position HP until it reaches a moving speed 3 (in this example, 50 mm/s, which is ⅛ of the moving speed 1) for reading the original document at a read magnification of 400%. Then, while the speed 3 is maintained, the carriage passes the image scan start position (that is, light irradiation start position) on the document table, and thus the image on the sheet placed on the document table is read.
When the distance from the home position to the image scan start position is set to 60 mm, the carriage reaches the image scan start position in 1205 ms as shown in FIG. 1.
As described above, the moving speed of the carriage increases from the home position HP and reaches the level which corresponds to a respective magnification for reading an original document. Then, the moving speed is maintained at that level to the image scan start position on the document table. For example, as the read magnification increases from 50%, the moving speed is decreased. More specifically, as compared to the case where the read magnification is 50%, the moving speed becomes ½ of that of the case where the read magnification is 100%, or it becomes ⅛ of that of the case where the magnification is 400%.
As a result, when the read magnification is increased, it takes more time to move the carriage from the home position HP to the image scan start position on the sheet table, that is, taking up a great amount of time in image scanning.